1. Field of the Invention
The present invention relates to a cathode ray tube having a shadow mask, which is used for a television receiver, a computer display, and the like.
2. Description of the Prior Art
FIG. 3 is a cross-sectional view illustrating an example of a conventional color cathode ray tube. The color cathode ray tube 1 shown in FIG. 3 includes a substantially rectangular-shaped face panel 2 having a phosphor screen 2a on its inner surface, a funnel 3 connected to the rear side of the face panel 2, an electron gun 4 contained in a neck portion 3a of the funnel 3, a shadow mask 6 facing the phosphor screen 2a inside the face panel 2, and a mask frame 7 for fixing the shadow mask 6. Furthermore, in order to deflect and scan electron beams, a deflection yoke 5 is provided on the outer periphery of the funnel 3.
The shadow mask 6 plays a role of selecting colors with respect to three electron beams emitted from the electron gun 4. xe2x80x9cAxe2x80x9d shows a track of the electron beams. The shadow mask has a flat plate provided with a number of substantially slot-shaped apertures formed by etching. The slot-shaped aperture is a through aperture through which electron beams pass.
In a color cathode ray tube, due to the thermal expansion caused by the impact of the emitted electron beams, the electron beam through aperture is shifted. Consequently, a doming phenomenon occurs. That is, the electron beams passing through the electron beam through apertures fail to hit a predetermined phosphor correctly, thus causing unevenness in colors. Therefore, a tension force to absorb the thermal expansion due to the temperature increase of the shadow mask is applied in advance, and then the shadow mask is stretched and held to the mask frame.
FIG. 4 is a perspective view illustrating an example of a conventional color-selecting electrode. A mask frame 10 is a rectangular frame and is made of a pair of long frame supports 11, facing each other, fixed to a pair of short frames made of elastic members 12. On a shadow mask 13, a number of substantially slot-shaped apertures 14 are formed as electron beam through apertures by etching and arranged in a number of lines. In this drawing, a tension method is employed and the shadow mask 13 is stretched and held between the supports 11 with a tension force applied mainly in the direction indicated by arrow Y.
When the shadow mask is stretched and held as mentioned above, even if the temperature of the shadow mask is raised, it is possible to reduce the amount of displacement between an aperture of the shadow mask and phosphor stripes of the phosphor screen.
However, the above-mentioned conventional color cathode ray tube suffers from the following problem. As shown in FIG. 4, the shadow mask 13 is stretched in the direction Y and fixed to the supports 11 in a state in which upper and lower ends of the shadow mask 13 are held. In this case, the shadow mask 13 that is stretched in the direction Y expands in the direction Y and also contracts in the direction indicated by arrow X that is perpendicular to the direction Y by an amount corresponding to the Poisson""s ratio.
With respect to an aperture line 15, the shadow mask 13 is stretched in a state in which its upper and lower ends are held, so that the aperture line 15 expands in the direction Y, and at the same time, the aperture line is curved toward the center of the shadow mask 13 as indicated by the double-dashed line 15a. 
When the color cathode ray tube is operated in this state, electron beams strike the surface of the shadow mask to reduce the tension force in the direction Y and also to reduce the compressive force in the direction X at the same time. As a result, the aperture line also returns to the outer (peripheral) direction. In other words, the problem with the conventional color cathode ray tube described above was that this shifting of the apertures due to the above-mentioned return movement caused a color displacement, unevenness in colors, and reduction in luminance.
It is an object of the present invention to solve the conventional problem described above by providing a cathode ray tube capable of reducing the shifting of apertures in the horizontal direction of the screen during the operation of the cathode ray tube, thereby preventing a color displacement, unevenness in colors, and reduction in luminance from occurring.
In order to achieve the above-mentioned object, a cathode ray tube according to the present invention includes a shadow mask made of a flat plate provided with a number of apertures and bridges for linking the apertures neighboring in the vertical direction, wherein, by taking a center line of the shadow mask in the horizontal direction as an X-axis and a center line of the shadow mask in the vertical direction as a Y-axis, the bridges in the vicinity of both ends in an X-axis direction of a portion where the apertures are formed have a vertical arrangement pitch that is greater than that of the bridges in the vicinity of the Y-axis. According to the cathode ray tube as described above, when a tension force is applied in the Y-axis direction to the shadow mask so that the shadow mask is stretched and held, a displacement in the X-axis direction of the aperture lines located in the vicinity of both the ends of the shadow mask in the X-axis direction can be suppressed to a small value. Therefore, the shifting of the apertures in the X-axis direction during the operation of the cathode ray tube can be reduced, thereby preventing a color displacement, unevenness in colors, and reduction in luminance from occurring and in addition, the occurrence of wrinkles in the shadow mask when the shadow mask is stretched and held also can be suppressed.
In the above-mentioned cathode ray tube, it is preferable that the arrangement pitch increases with approach to both the ends in the X-axis direction.
Furthermore, it is preferable that the arrangement pitch is substantially the same up to predetermined positions in the X-axis direction and increases with approach to both the ends in the X-axis direction from the predetermined positions.
Furthermore, it is preferable that the arrangement pitch is substantially the same up to predetermined positions in the X-axis direction and increases stepwise for every aperture line of a constant number with approach to both the ends in the X-axis direction from the predetermined positions.
Still further, it is preferable that the predetermined positions are located in the vicinity of both the ends of the X-axis direction.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.